1. Field of the Invention
The present invention relates to a stabilizer control apparatus for a vehicle, and more particularly to an apparatus for controlling a torsional rigidity of a stabilizer disposed between a right wheel and a left wheel.
2. Description of the Related Arts
In general, a stabilizer control apparatus for a vehicle is provided for applying an appropriate roll moment to a vehicle by means of a stabilizer, while the vehicle is performing a turning operation, to reduce or restrict a rolling motion of the vehicle body. In order to achieve this function, Japanese Patent Laid-open Publication No. 2000-71739, for example, discloses an apparatus for controlling efficiency of a stabilizer to vary an apparent torsional rigidity of the stabilizer by driving and controlling an actuator in response to output of means for detecting a turning level of a vehicle, e.g., a lateral acceleration sensor. According to that apparatus, the actuator is actuated to control a varying position of the vehicle, so as to provide a property attaching importance to ride comfort in a range of relatively small turning operation, and it is actuated to control the varying position of the vehicle, so as to provide a property attaching importance to stability, with an inherent rigidity of the stabilizer produced by limiting the movement of the actuator with a small movable range, when the turning operation is increased.
In Japanese Patent Laid-open Publication No. 9-20223, there is disclosed a method for determining a rough road, by calculating a wheel acceleration on the basis of a wheel speed, and obtaining its high-frequency component through a high-pass filter, to calculate a dispersion of the wheel acceleration, on the basis of which the rough road is determined. Also, Japanese Patent Laid-open Publication No. 2001-63544 discloses a method for determining a rough road according to a result of F-study through a dispersion of differentiated value of output from an acceleration sensor obtained when a vehicle is running on a reference flat road surface at a small steering angle, and a dispersion of differentiated value of output from the acceleration sensor obtained when the vehicle is running on the present road surface at the small steering angle. Furthermore, in Japanese Patent Laid-open Publication No. 9-193776, as to factors for use in a vehicle stability control, there are disclosed spinning value indicative of spinning state variable and drift-out value indicative of drift-out state variable.
According to the apparatus for controlling efficiency of the stabilizer as described in the Japanese Publication No. 2000-71739, however, the lateral acceleration indicative of the turning level is not caused only by the turning operation of the vehicle, but it may be caused by a rough surface of a road, on which the vehicle is running. For example, the lateral acceleration may be caused by the rough surface of the road, even if the vehicle is moving straight, as shown in FIG. 5, in the case where the vehicle is running on such a road surface called as a roll-sweep that the road has a property of opposite phases at its right side and left side of a moving direction of the vehicle, with frequency and amplitude of the property being varied as time passes.
Basically, the stabilizer control apparatus is provided for controlling to restrain a roll angle of vehicle body caused at the time of turning operation of the vehicle. This can be achieved by controlling and actuating a stabilizer actuator to increase a torsional rigidity of a stabilizer in response to a lateral acceleration indicative of the turning operation. On the other hand, in order to improve the ride comfort, it is required to lower the torsional rigidity of the stabilizer, so as to prevent irregularity of the road surface from being transmitted to the vehicle body. Therefore, if the lateral acceleration is caused by the rough road surface, as described above, the stabilizer is controlled to increase its torsional rigidity in response to variation of the lateral acceleration. As a result, the stabilizer control will be made so as to act against change in rolling motion caused only by the rough road surface, while the vehicle is not in its turning operation, which control might result in deteriorating the ride comfort.